Overcoating of Reflection Gratings

Most standard reflection gratings are furnished with an aluminum (Al) reflecting surface. While no other metal has more general application, there are several special situations where alternative surfaces or coatings are recommended. Gratings coated with gold (Au) and silver (Ag) have been used for some time for higher reflectivity in certain spectral regions, as have more exotic materials such as iridium (Ir), osmium (Os) and platinum (Pt).

The reflectivity of aluminum drops rather sharply for wavelengths below 170 nm. While freshly deposited, fast-fired pure aluminum in high vacuum maintains its reflectivity to wavelengths shorter than 100 nm, the thin layer of oxide that grows on the aluminum (upon introduction of the coating to atmosphere) will cause a reduction in efficiency below about 250 nm. Fortunately, a method borrowed from mirror technology makes it possible to preserve the reflectivity of aluminum to shorter wavelengths.96 The process involves overcoating the grating with a thin layer of fast-fired aluminum, which is followed immediately by a coating of magnesium fluoride (MgF₂) approximately 25 nm thick. The main purpose of the MgF₂ coating is to protect the aluminum from oxidation. The advantage of this coating is especially marked in the region between 120 and 200 nm. While reflectivity drops off sharply below this region, it remains higher than that of gold and comparable to that of platinum, the most commonly used alternative materials, down to 70 nm.

Overcoating gratings so that their surfaces are coated with two layers of different metals sometimes leads to a change in diffraction efficiency over time. Hunter et al. have found the cause of this change to be intermetallic diffusion. For example, they measured a drastic decrease (over time) in efficiency at 122 nm for gratings coated in Au and then overcoated in Al + MgF₂; this decrease was attributed to the formation of intermetallic compounds, primarily AuAl₂ and Au₂Al. Placing a suitable dielectric layer such as SiO between the two metallic layers prevents this diffusion.

As mentioned elsewhere, fingerprints are a danger to aluminized optics. It is possible to overcoat such optics, both gratings and mirrors, with dielectrics like MgF₂, to prevent finger acids from attacking the aluminum. These MgF₂ coatings cannot be baked, as is customary for glass optics, and therefore must not be cleaned with water. Spectrographic-grade organic solvents are the only recommended cleaning agents, and they should be used sparingly and with care.

Single-layer and multilayer dielectric coatings, which are so useful in enhancing plane mirror surfaces, are less generally applicable to diffrac-tion gratings, since in certain circumstances these coatings lead to complex guided wave effects. For wavelengths below 30 nm, in which grazing angles of incidence and diffraction are common, multilayer coatings can enhance efficiency considerably.